The violin is sobbing, and not in the good way. It sounds as though the poor thing's voice is about to give out entirely under the torture Sherlock is inflicting on it. It's the sound of razor blades being taken to a man's mind, of thoughts devouring themselves in cannibalistic fury.
It is 4 am.
John pads downstairs to watch the lean body sway to its own frenzied rhythms under the sulphurous glow of the London night. Sherlock's eyes are closed, and John is glad, because right now he would burn alive under their intensity. He listens to the sounds of Sherlock's brain and doesn't interrupt till the music pauses.
"Where can a man go in London to hear himself think?" he asks then, and holds out Sherlock's coat.
Sherlock shows him.